1. Field of the Invention
The present invention relates to a digital broadcast receiving apparatus and a control method thereof, more specifically, relates to the improvement of the synchronization of the displayed image and the reproduced sound.
2. Description of the Related Art
As is well-known, in the above-mentioned digital television broadcast, digitized image data and sound data are broadcasted after they are respectively subjected to compression coding according to MPEG (Moving Picture Image Coding Experts Group), and then converted into the transport stream form.
The receiving apparatus executes MPEG decoding respectively for the received image data component and sound data component converted into the transport stream to obtain the original digital image data and sound data, thereafter convert them analog data for image and sound reproduction.
In the MPEG decoding process, the period of time required to MPEG decode the image data and that required for the sound data are different from each other due to the difference in the amounts of the two types of data. If the image and sound are displayed/reproduced directly after MPEG decoding, a time lag will occur between the displayed image and the reproduced sound due to this difference in data processing time.
In order to solve this problem, the synchronization of the displayed image and the reproduced sound, i.e. so-called lip-sync process, has been conventionally executed with use of a PTS (Presentation Time Stamp) contained in the image and sound data subjected to MPEG compression, as suggested in Jpn. Pat. Appln. KOKAI Publication No. 7-177479 or the like.
On the other hand, data processing and surround sound processing such as graphic processing or format conversion, for the image and sound data after MPEG decoding, are recently performed in accordance with the currently distributed multi-function receiving apparatuses.
The technique for synchronizing the displayed image and the reproduced sound with use of the PTS is, however, intended to do so by adjusting the time lag of the image and sound data after MPEG decoding.
Therefore, if various data processings are performed on the image and sound data after MPEG decoding and a time lag occurs thereby, the displayed image and the reproduced sound cannot be synchronized merely by use of PTS.
Other than the technique taught in the above-mentioned patent application, techniques of synchronizing the displayed image and the reproduced sound are suggested in Jpn. Pat. Appln. KOKAI Publication No. 6-343165 and Jpn. Pat. Appln. KOKAI Publication No. 9-247563.
However, the former technique is intended to attain lip-sync by adjusting the delay time of the image data with use of a flag added to the sound data as the head of the field such that the phases of the sound data and the image data are synchronized with each other. Similarly, the latter is intended to attain lip-sync by adjusting the period of the image reproduction time on the basis of the adjusted sound speed and the partition. As should be clear from the above, neither of the techniques relate to the above-mentioned problem or suggest any solution thereto.